The invention relates to an air filter medium in the form of a layer-like non-woven material. Such filter media are used in filters, for example in indoor air filters and air conditioning systems, in particular however in air filters for the motor vehicle interior or for engines.
The filter media are produced in a melting and spinning method such as a spunbond method or a melt-blown method as is described for example in DE 41 23 122 A1.
The intake air of internal combustion engines for example in motor vehicles or in the off-road area is usually filtered to protect the engine combustion chamber from mechanical damage owing to sucked-in particles from the ambient air. An important criterion in the design of the filter elements is to ensure a long service life of the filter while at the same time separating out a high proportion of the sucked-in particles.
Motor vehicles have a precisely calculated energy distribution system. Only limited amounts of energy are available to the heating/ventilation/air-conditioning area. The costs of vehicle components must also vary only within a very narrow range. On the other hand, car buyers are making greater and greater demands in terms of comfort and safety. In consideration of these aspects, particle filters with the smallest possible fall in pressure or differential pressure are of particular significance, as only a low pressure has to be generated by the fan motor and energy consumption is therefore low. Furthermore, the latter operates more quietly owing to the lower power needed, as a result of which the noise is reduced and therefore driving comfort is substantially increased.
The demand for filter systems with low differential pressure competes with the demanded separation performance and service life, that is, the time expressed in mileage that a filter can remain in the vehicle until it must be replaced.
For example, pollen filters which only filter pollen out of the inflowing air in the vehicle are not sufficient for the vehicle interior. The allergens to which the immune system of allergic persons reacts are proteins, the diameter of which is only a fraction of the diameter of pollen. They are in a size range of around 0.1 μm, that is, the range which has the greatest problems for particle filters, what is known as the MPPS (most penetrating particle size). Correspondingly, the separation performance in this size range should be at least 50% and is measured by means of an aerosol, the particles of which have approximately the same density, for example NaCl. At the same time, service lives of at least 30,000 kilometers can be achieved with such filters when installed in motor vehicles.
EP 1 198 279 B1 discloses a melt-blown non-woven fabric which consists of at least two layers, one layer consisting of microfibres while the other layer consists of macrofibres. The layers are layered on top of each other and adhere to each other by means of connections between the fibres. However, if non-woven fabrics of this type in the field of air filtration, they become clogged very quickly owing to the particles present in the air. A fundamental difference is drawn between two types of filter and filter processes.
Surface filters have a relatively smooth, dense filter medium, which results in a filter cake building up on the surface on the inflow side of the filter medium, which filter cake supports the filter effect within a short time. However, the loss in pressure, i.e. the differential pressure, across the filter medium increases rapidly. If a critical value is reached, a reverse pulse is usually output onto the medium in the direction counter to the normal air flow direction, so the filter cake largely falls off the medium (except for a little residue) and the filter medium is virtually regenerated. The pulses are often repeatable, but the efficiency of the regeneration is reduced over time, so the filter must be replaced.
Depth filters have rather open-pored fibre material, which picks up dirt particles over the entire depth of the medium. This is often constructed three-dimensionally, i.e. there are coarse fibre diameters on the inflow side and fine ones on the outflow side. The dirt particles pass into the medium and are arrested and retained upstream of the fine fibre layer. The pressure loss of this filter medium increases only slowly. However, it is not possible to clean it off, as the dirt particles are embedded in the open-pored fibre material.
There is thus a conflict between two effects: Either a filter is chosen which builds up a high differential pressure within a short time and therefore must be cleaned off often (surface filter) or a filter is chosen which can absorb more particles before the differential pressure reaches a critical value but cannot be cleaned off (depth filter).
The object of the present invention is to overcome the stated disadvantages and provide an air filter medium which is easy to produce and cost-effective.